Semiconductor device including channel formation region including oxide semiconductor

ABSTRACT

A first insulating film in contact with an oxide semiconductor film and a second insulating film are stacked in this order over an electrode film of a transistor including the oxide semiconductor film, an etching mask is formed over the second insulating film, an opening portion exposing the electrode film is formed by etching a portion of the first insulating film and a portion of the second insulating film, the opening portion exposing the electrode film is exposed to argon plasma, the etching mask is removed, and a conductive film is formed in the opening portion exposing the electrode film. The first insulating film is an insulating film whose oxygen is partly released by heating. The second insulating film is less easily etched than the first insulating film and has a lower gas-permeability than the first insulating film.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a manufacturing method of asemiconductor device. A semiconductor device refers to a semiconductorelement itself or a device including a semiconductor element. As anexample of such a semiconductor element, for example, a transistor canbe given. A semiconductor device also refers to a display device such asa liquid crystal display device.

2. Description of the Related Art

In recent years, semiconductor devices have been necessary for humanlife. Transistors included in such semiconductor devices aremanufactured by forming a film over a substrate and processing the filminto a desired shape by etching or the like.

In such a manufacturing method of a semiconductor device, for example, afirst conductive film is formed, an insulating film is formed to coverthe first conductive film, an opening portion overlapping with the firstconductive film is formed in the insulating film, and a secondconductive film which is connected to the first conductive film throughthe opening portion may be formed. In such a manner, a plurality ofwirings provided using different conductive films is arranged verticallyand horizontally; thus an element substrate on which a plurality ofelements is arranged in a matrix can be manufactured.

Not only the cross-sectional shape of such an opening portion, but alsothe state of the surface of the opening portion depends on etchingconditions. For example, etching residues exist in the opening portionin many cases. Such an etching residue existing in the opening portionmay inhibit a connection of a plurality of conductive films in somecases. As a method for removing such a residue in the opening portion,reverse sputtering can be given (e.g., Patent Document 1).

On the other hand, in recent years, metal oxides having semiconductorcharacteristics (hereinafter, referred to as oxide semiconductors) haveattracted attention. Oxide semiconductors can be applied to transistors(Patent Documents 2 and 3).

REFERENCE

-   [Patent Document 1] Japanese Published Patent Application No.    2001-305576-   [Patent Document 2] Japanese Published Patent Application No.    2007-123861-   [Patent Document 3] Japanese Published Patent Application No.    2007-096055

SUMMARY OF THE INVENTION

A transistor in which an oxide semiconductor film is used for a channelformation region has favorable electric characteristics such asextremely higher field-effect mobility than a semiconductor device inwhich a silicon film is used for a channel formation region. However, atpresent, such a semiconductor device has a problem in reliability. Asone of factors of decrease in the reliability of a semiconductor device,water and hydrogen contained in an oxide semiconductor film can begiven. Therefore, it is important to reduce the amount of water andhydrogen contained in an oxide semiconductor film as much as possible.

In addition, when many oxygen vacancies are contained in the oxidesemiconductor film used in such a transistor, the resistance of a regionwhere the oxygen vacancies exist is reduced, which causes leakagecurrent between a source and a drain. Accordingly, oxygen vacancies inthe oxide semiconductor film are preferably reduced. In order to reduceoxygen vacancies, enough oxygen is preferably supplied from the outside.

One of methods for reducing the amount of water and hydrogen in an oxidesemiconductor film and supplying oxygen is, for example, heat treatmentwhich is performed in a state where a base film is provided in contactwith the oxide semiconductor film. By heat treatment in a state wherethe amount of water and hydrogen in the base film is reduced and theamount of oxygen contained in the base film is increased, oxygen can besupplied to the oxide semiconductor film while entry of water andhydrogen into the oxide semiconductor film is suppressed.

In order to reduce the amount of water and hydrogen in the base film,heat treatment may be performed after formation of the base film by aCVD method, a sputtering method, or the like before formation of theoxide semiconductor film. However, when heat treatment is performed,oxygen is also released together with water and hydrogen.

In order to prevent such release of oxygen, an aluminum oxide film maybe provided as a barrier film in heat treatment. However, it takes along time to etch the aluminum oxide film, because aluminum oxide haspoor chemical reactivity. Therefore, in the case where an insulatingfilm in which an opening portion is formed is a multi-layer film, andpart of the layers is formed using aluminum oxide, it takes a long timeto form the opening portion, so that many etching residues are generatedin the opening portion. Such etching residues, in the case where theopening portion is a contact hole, may cause an increase in contactresistance.

In one embodiment of the present invention, it is an object to provide asemiconductor device having a high reliability, in which leakage currentbetween a source and a drain does not easily flow, and in which acontact resistance is low and to provide a manufacturing method thereof.

Another embodiment of the present invention is a manufacturing method ofa semiconductor device including the steps of forming a transistor inwhich a channel formation region is formed in an oxide semiconductorfilm, forming a first insulating film provided in contact with the oxidesemiconductor film and a second insulating film in this order stackedover an electrode film of the transistor, forming an etching maskincluding an opening portion over the second insulating film, forming anopening portion exposing the electrode film by etching a portion of thefirst insulating film and a portion of the second insulating film, whichoverlap with the opening portion of the etching mask, exposing theopening portion of the first insulating film and the second insulatingfilm to argon plasma, removing the etching mask, and forming aconductive film in the opening portion of the first insulating film andthe second insulating film. The first insulating film is an insulatingfilm in which part of oxygen is released by heating, and the secondinsulating film is less easily etched than the first insulating film andhas a higher gas barrier property than the first insulating film.

Another embodiment of the present invention is a manufacturing method ofa semiconductor device including the steps of forming a transistor inwhich a channel formation region is formed in an oxide semiconductorfilm, forming a first insulating film provided in contact with the oxidesemiconductor film and a second insulating film in this order stackedover an electrode film of the transistor, forming an etching maskincluding an opening portion over the second insulating film, forming anopening portion exposing the electrode film by etching a portion of thefirst insulating film and the second insulating film, which overlap withthe opening portion of the etching mask, removing the etching mask,subjecting the opening portion of the first insulating film and thesecond insulating film to reverse sputtering, and forming a conductivefilm in the opening portion of the first insulating film and the secondinsulating film by a sputtering method. The first insulating film is aninsulating film in which part of oxygen is released by heating, and thesecond insulating film is less easily etched than the first insulatingfilm and has a higher gas barrier property than the first insulatingfilm.

In the above-described structures, the first insulating film ispreferably a silicon oxide film, a silicon oxynitride film, or a siliconnitride oxide film, and the second insulating film is preferably analuminum oxide film.

Another embodiment of the present invention is a manufacturing method ofa semiconductor device including the steps of forming a transistor inwhich a channel formation region is formed in an oxide semiconductorfilm, forming a first insulating film provided in contact with the oxidesemiconductor film, a second insulating film, and a third insulatingfilm in this order stacked over an electrode film of the transistor,forming an etching mask including an opening portion over the thirdinsulating film, forming an opening portion exposing the electrode filmby etching a portion of the first insulating film, a portion of thesecond insulating film, and a portion of the third insulating film,which overlap with the opening portion of the etching mask, exposing theopening portion of the first insulating film, the second insulatingfilm, and the third insulating film to argon plasma, removing theetching mask, and forming a conductive film in the opening portion ofthe first insulating film, the second insulating film, and the thirdinsulating film. The first insulating film is an insulating film inwhich part of oxygen is released by heating, and the second insulatingfilm is less easily etched than the first insulating film and has ahigher gas barrier property than the first insulating film.

Another embodiment of the present invention is a manufacturing method ofa semiconductor device including the steps of forming a transistor inwhich a channel formation region is formed in an oxide semiconductorfilm, forming a first insulating film provided in contact with the oxidesemiconductor film, a second insulating film, and a third insulatingfilm in this order stacked over an electrode film of the transistor,forming an etching mask including an opening portion over the thirdinsulating film, forming an opening portion exposing the electrode filmby etching a portion of the first insulating film, the second insulatingfilm, and the third insulating film, which overlap with the openingportion of the etching mask, removing the etching mask, subjecting theopening portion of the first insulating film, the second insulatingfilm, and the third insulating film to a reverse sputtering, and forminga conductive film in the opening portion of the first insulating film,the second insulating film, and the third insulating film by asputtering method. The first insulating film is an insulating film inwhich part of oxygen is released by heating, and the second insulatingfilm is less easily etched than the first insulating film and has ahigher gas barrier property than the first insulating film.

In the above-described structures, the first insulating film and thethird insulating film are each preferably a silicon oxide film, asilicon oxynitride film, or a silicon nitride oxide film, and the secondinsulating film is preferably an aluminum oxide film.

In the above-described structures, the third insulating film ispreferably thicker than the first insulating film.

In this specification, an “electrode film” refers to a conductive filmto be a gate electrode, a source electrode, or a drain electrodecollectively. That is, an “electrode film” refers to a conductive filmto be a gate electrode, a source electrode, or a drain electrode, andthe function is not limited.

In this specification, a “gas barrier property” refers to a propertywith respect to at least oxygen gas, and “high gas barrier property”means low oxygen gas-permeability, and “low gas barrier property” meanshigh oxygen gas-permeability.

In this specification, an “etching mask” refers to a mask formed forpreventing etching of a film formed below the etching mask. As anetching mask, a resist mask can be given, for example.

According to one embodiment of the present invention, a semiconductordevice having a high reliability, in which leakage current between asource and a drain does not easily flow, and in which a contactresistance is low can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1C illustrate a method for manufacturing a semiconductordevice according to one embodiment of the present invention;

FIGS. 2A to 2C illustrate a method for manufacturing a semiconductordevice according to one embodiment of the present invention;

FIGS. 3A to 3C illustrate a method for manufacturing a semiconductordevice according to one embodiment of the present invention;

FIGS. 4A to 4C illustrate a method for manufacturing a semiconductordevice according to one embodiment of the present invention;

FIGS. 5A to 5C illustrate a method for manufacturing a semiconductordevice according to one embodiment of the present invention;

FIGS. 6A to 6E illustrate a structure of an oxide material which can beapplied to one embodiment of the present invention;

FIGS. 7A to 7C illustrate a structure of an oxide material which can beapplied to one embodiment of the present invention;

FIGS. 8A to 8C illustrate a structure of an oxide material which can beapplied to one embodiment of the present invention;

FIGS. 9A to 9C illustrate a method for manufacturing a semiconductordevice according to one embodiment of the present invention;

FIGS. 10A to 10C each illustrate a method for manufacturing asemiconductor device according to one embodiment of the presentinvention; and

FIGS. 11A and 11B are cross-sectional STEM images showing openings ofComparative Sample and Example Sample.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings. However, the presentinvention is not limited to the following description and it is easilyunderstood by those skilled in the art that the mode and details can bevariously changed without departing from the scope and spirit of thepresent invention. Accordingly, the present invention should not beconstrued as being limited to the description of the embodiments below.

Embodiment 1

In this embodiment, a manufacturing method of a semiconductor devicewhich is one embodiment of the present invention will be described. Inthis embodiment, a stacked-layer insulating film in which an openingportion is formed has a two-layer structure.

First, a base insulating film 102 and a first conductive film 104 areformed over a substrate 100, and a first etching mask 106 is formed overthe first conductive film 104 (FIG. 1A).

The substrate 100 is not limited to a certain type, and has enough heatresistance and chemical resistance in a process of manufacturing asemiconductor device. As the substrate 100, a glass substrate, a quartzsubstrate, a ceramic substrate, a plastic substrate, and the like can begiven. For a plastic substrate, a plastic material having low refractiveindex anisotropy is preferably used. As the plastic material having lowrefractive index anisotropy, polyether sulfone, polyimide, polyethylenenaphthalate, polyvinyl fluoride, polyester, polycarbonate, an acrylicresin, a prepreg, and the like can be given.

The base insulating film 102 may be formed using an insulating oxidewhich functions as an oxygen supply source. Therefore, the baseinsulating film 102 may be formed using an insulating oxide in whichmore oxygen than that in the stoichiometric proportion is contained andpart of the oxygen is released by heat treatment. As an example of suchan insulating oxide, silicon oxide represented by SiO_(x) where x>2 canbe given.

However, a material of the base insulating film 102 is not limitedthereto, and the base insulating film 102 may be formed using siliconoxide, silicon oxynitride, silicon nitride oxide, aluminum oxide,aluminum oxynitride, gallium oxide, hafnium oxide, yttrium oxide, or thelike.

Note that “silicon nitride oxide” contains more nitrogen than oxygen and“silicon oxynitride” contains more oxygen than nitrogen.

The base insulating film 102 may have a single-layer structure or astacked-layer structure of a plurality of layers. For example, as thestacked-layer structure of a plurality of layers, a stacked structure inwhich a silicon oxide film is formed over a silicon nitride film isgiven.

The thickness of the base insulating film 102 is greater than or equalto 50 nm, preferably greater than or equal to 200 nm and less than orequal to 500 nm. In particular, when the thickness is increased withinthe above range, much oxygen can be diffused into the oxidesemiconductor film by heat treatment and defects (oxygen vacancies) atthe interface between the base insulating film 102 and the oxidesemiconductor film can be reduced, which is preferable.

In an insulating oxide which contains more oxygen than thestoichiometric proportion, part of the oxygen is easily released by heattreatment. The release amount of oxygen (the value converted into thatof oxygen atoms) obtained by TDS analysis when part of oxygen is easilyreleased by heat treatment is greater than or equal to 1.0×10¹⁸atoms/cm³, preferably greater than or equal to 1.0×10²⁰ atoms/cm³, morepreferably greater than or equal to 3.0×10²⁰ atoms/cm³.

Here, the release amount of oxygen in the TDS analysis (the valueconverted into the number of oxygen atoms) is measured with a thermaldesorption spectroscopy apparatus produced by ESCO Ltd., EMD-WA1000S/W,and calculated using a silicon wafer containing hydrogen atoms at 1×10¹⁶atoms/cm³ as the standard sample.

Here, the TDS analysis is described. The release amount of a gas in theTDS analysis is proportional to a time integration value of ionintensity. Thus, the release amount of a gas can be calculated from thetime integral value of the ion intensity of an insulating oxide and areference value of a standard sample. The reference value of a standardsample refers to the ratio of the density of a predetermined atomcontained in a sample (standard sample) to the integral value of aspectrum.

For example, by using the ion intensity of a silicon wafer containing apredetermined density of hydrogen, which is a standard sample, and theion intensity of an insulating oxide, the release amount (N_(O2)) ofoxygen molecules (O₂) of the insulating oxide can be obtained by thefollowing formula (1).(FORMULA 1)N_(O2)═N_(H2)/S_(H2)×S_(O2)×α  (1)

N_(H2) is a value obtained by conversion of the number of hydrogenmolecules (H₂) released from the standard sample into density. S_(H2) isa value obtained by time integration of the ion intensity of hydrogenmolecules (H₂) of the standard sample. In other words, the referencevalue of the standard sample is N_(H2)/S_(H2). S_(O2) is a valueobtained by time integration of the ion intensity of oxygen molecules(O₂) of the insulating oxide. α is a coefficient affecting the ionintensity. Refer to Japanese Published Patent Application No. H6-275697for details of Formula 1.

Note that in the TDS analysis, oxygen is partly detected as an oxygenatom. The ratio between oxygen molecules and oxygen atoms can becalculated from the ionization rate of the oxygen molecules. Note that,since the coefficient α includes the ionization rate of the oxygenmolecules, the number of the released oxygen atoms can also becalculated through the evaluation of the number of the released oxygenmolecules.

Note that N_(O2) is the release amount of oxygen molecules (O₂);therefore, the release amount of oxygen converted into oxygen atoms istwice the release amount of oxygen molecules (O₂).

The base insulating film 102 may be formed by a sputtering method, a CVDmethod, or the like. In the case of using a CVD method, hydrogen or thelike is preferably released and removed from the formed base insulatingfilm 102 by heat treatment.

In the case where the base insulating film 102 is formed using a siliconoxide by a sputtering method, a quartz (preferably, synthesized quartz)target may be used as a target, and an argon gas may be used as asputtering gas. Alternatively, a silicon target and a gas containingoxygen may be used as a target and a sputtering gas, respectively. Asthe gas containing oxygen, only an oxygen gas may be used or a mixed gasof an argon gas and an oxygen gas may be used.

The first conductive film 104 may be formed using a conductive material.Here, as a conductive material, a metal such as aluminum, chromium,copper, tantalum, titanium, molybdenum, tungsten, manganese, magnesium,beryllium, or zirconium or an alloy containing one or more of the abovemetals as a component can be given. The first conductive film 104 mayhave a single-layer structure or a stacked-layer structure.

Note that the first conductive film 104 is preferably formed usingcopper because the resistance of a wiring formed using the firstconductive film 104 can be reduced. Here, in the case where the firstconductive film 104 has a stacked structure, at least one layer of thefirst conductive film 104 is formed using copper.

Alternatively, the first conductive film 104 may be formed using alight-transmitting conductive material such as indium tin oxide, indiumoxide containing tungsten oxide, indium zinc oxide containing tungstenoxide, indium oxide containing titanium oxide, indium tin oxidecontaining titanium oxide, or indium tin oxide to which indium zincoxide or silicon oxide is added.

Alternatively, the first conductive film 104 may be formed by stacking afilm of the light-transmitting conductive material and a film of themetal.

The thickness of the first conductive film 104 may be, for example,greater than or equal to 100 nm and less than or equal to 300 nm.

The first conductive film 104 may be formed by a sputtering method, aCVD method, or the like.

The first etching mask 106 may be formed of a resist material. Note thatthere is no limitation thereto as long as it functions as a mask whenthe first conductive film 104 is processed.

Here, after the base insulating film 102 is formed, heat treatment whichremoves hydrogen from the base insulating film 102 is preferablyperformed before the first conductive film 104 is formed. Alternatively,after the first conductive film 104 is formed, heat treatment whichremoves hydrogen from the base insulating film 102 may be performedbefore the first etching mask 106 is formed.

Next, the first conductive film 104 is processed with the use of thefirst etching mask 106, so that the first conductive film 108 is formed(FIG. 1B).

The first conductive film 104 may be processed by dry etching. Forexample, a chlorine gas or a mixed gas of a boron trichloride gas and achlorine gas can be given as an etching gas used for the dry etching.However, there is no limitation thereto; wet etching may be used oranother method may be used.

The first conductive film 108 functions at least as a source electrodeand a drain electrode.

Next, the first etching mask 106 is removed, and an oxide semiconductorfilm 110 is formed over and in contact with the base insulating film 102and the first conductive film 108 (FIG. 1C).

Here, after the first etching mask 106 is removed and before the oxidesemiconductor film 110 is formed, heat treatment which removes hydrogenfrom the base insulating film 102 may be performed.

In the case where the first etching mask 106 is formed using a resistmaterial, the first etching mask 106 may be removed by ashing usingoxygen plasma. Alternatively, a resist stripper may be used. Further,alternatively, both of them may be used.

The oxide semiconductor film 110 preferably contains at least indium(In) or zinc (Zn). In particular, both In and Zn are preferablycontained.

In addition, one or a plurality of elements selected from gallium (Ga),tin (Sn), hafnium (Hf), and aluminum (Al) is preferably contained as astabilizer for reducing variations in electrical characteristics of thetransistor including the oxide semiconductor film 110.

As another stabilizer, one or plural kinds of lanthanoid such aslanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium(Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy),holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), or lutetium(Lu) may be contained.

As the oxide semiconductor, for example, an indium oxide, a tin oxide, azinc oxide, a two-component metal oxide such as an In—Zn-based oxide, aSn—Zn-based oxide, an Al—Zn-based oxide, a Zn—Mg-based oxide, aSn—Mg-based oxide, an In—Mg-based oxide, or an In—Ga-based oxide, athree-component metal oxide such as an In—Ga—Zn-based oxide (alsoreferred to as IGZO), an In—Al—Zn-based oxide, an In—Sn—Zn-based oxide,a Sn—Ga—Zn-based oxide, an Al—Ga—Zn-based oxide, a Sn—Al—Zn-based oxide,an In—Hf—Zn-based oxide, an In—La—Zn-based oxide, an In—Ce—Zn-basedoxide, an In—Pr—Zn-based oxide, an In—Nd—Zn-based oxide, anIn—Sm—Zn-based oxide, an In—Eu—Zn-based oxide, an In—Gd—Zn-based oxide,an In—Tb—Zn-based oxide, an In—Dy—Zn-based oxide, an In—Ho—Zn-basedoxide, an In—Er—Zn-based oxide, an In—Tm—Zn-based oxide, anIn—Yb—Zn-based oxide, or an In—Lu—Zn-based oxide, a four-component metaloxide such as an In—Sn—Ga—Zn-based oxide, an In—Hf—Ga—Zn-based oxide, anIn—Al—Ga—Zn-based oxide, an In—Sn—Al—Zn-based oxide, anIn—Sn—Hf—Zn-based oxide, or an In—Hf—Al—Zn-based oxide can be used.

Note that here, for example, an In—Ga—Zn-based oxide means an oxidecontaining In, Ga, and Zn, and there is no limitation on the compositionratio of In, Ga, and Zn. Further, the In—Ga—Zn-based oxide semiconductormay contain a metal element other than In, Ga, and Zn.

For example, an In—Ga—Zn-based oxide with an atomic ratio ofIn:Ga:Zn=1:1:1 (=1/3:1/3:1/3) or In:Ga:Zn=2:2:1 (=2/5:2/5:1/5), or anoxide with an atomic ratio close to the above atomic ratios can be used.Alternatively, an In—Sn—Zn-based oxide with an atomic ratio ofIn:Sn:Zn=1:1:1 (=1/3:1/3:1/3), In:Sn:Zn=2:1:3 (=1/3:1/6:1/2), orIn:Sn:Zn=2:1:5 (=1/4:1/8:5/8), or an oxide with an atomic ratio close tothe above atomic ratios may be used.

However, without limitation to the materials given above, a materialwith an appropriate composition may be used depending on neededsemiconductor characteristics (e.g., mobility, threshold voltage, andvariation). In order to obtain necessary semiconductor characteristics,it is preferable that the carrier density, the impurity concentration,the defect density, the atomic ratio of a metal element to oxygen, theinteratomic distance, the density, or the like of the oxidesemiconductor film be set to be appropriate.

The oxide semiconductor film 110 may be either single crystal ornon-single-crystal. In the case of non-single-crystal, the oxidesemiconductor layer may be either amorphous or polycrystalline. Further,the oxide semiconductor may have a structure which includes crystallineportions in an amorphous portion, or the oxide semiconductor may benon-amorphous.

Here, one preferable mode of the oxide semiconductor film 110 will bedescribed. One preferable mode of the oxide semiconductor film 110 is anoxide including a crystal with c-axis alignment (also referred to asc-axis aligned crystal (CAAC)), which has a triangular or hexagonalatomic arrangement when seen from the direction of an a-b plane, asurface, or an interface. In the crystal, metal atoms are arranged in alayered manner, or metal atoms and oxygen atoms are arranged in alayered manner along the c-axis, and the direction of the a-axis or theb-axis of one crystal is different from those of another crystal in thea-b plane (the crystal rotates around the c-axis). Hereinafter, such anoxide is simply referred to as CAAC.

The CAAC means a non-single-crystal oxide including a phase that has atriangular, hexagonal, regular triangular or regular hexagonal atomicarrangement when seen from the direction perpendicular to the a-b planeand in which metal atoms are arranged in a layered manner or metal atomsand oxygen atoms are arranged in a layered manner when seen from thedirection perpendicular to the c-axis direction.

The CAAC is not a single crystal, but does not consist of only anamorphous state. Although the CAAC includes a crystallized portion(crystalline portion), a boundary between one crystalline portion andanother crystalline portion is not clear in some cases.

In the case where oxygen is included in the CAAC, nitrogen may besubstituted for part of oxygen included in the CAAC. The c-axes ofindividual crystalline portions included in the CAAC may be aligned inone direction (e.g., a direction perpendicular to a surface of asubstrate over which the CAAC is formed or a surface of the CAAC).Alternatively, the normals of the a-b planes of the individualcrystalline portions included in the CAAC may be aligned in onedirection (e.g., a direction perpendicular to a surface of a substrateover which the CAAC is formed or a surface of the CAAC).

The CAAC may be a conductor, a semiconductor, or an insulator. Further,the CAAC may or may not transmit visible light.

As an example of such a CAAC, there is an oxide which is formed into afilm shape and has a triangular or hexagonal atomic arrangement whenobserved from the direction perpendicular to a surface of the film or asurface of a formed substrate, and in which metal atoms are arranged ina layered manner or metal atoms and oxygen atoms (or nitrogen atoms) arearranged in a layered manner when a cross section of the film isobserved.

The CAAC will be described in FIGS. 6A to 6E, FIGS. 7A to 7C, and FIGS.8A to 8C. In FIGS. 6A to 6E, FIGS. 7A to 7C, and FIGS. 8A to 8C, thevertical direction basically corresponds to the c-axis direction and aplane perpendicular to the c-axis direction basically corresponds to thea-b plane. In the case where the expressions “an upper half” and “alower half” are simply used, boundary is the a-b plane. Further, inFIGS. 6A to 6E, O surrounded by a circle represents tetracoordinate Oand O surrounded by a double circle represents tricoordinate O.

FIG. 6A illustrates a structure including one hexacoordinate In atom andsix tetracoordinate oxygen atoms (tetracoordinate O atoms) close to theIn atom. A structure in which one metal atom and oxygen atoms close tothe metal atom are only illustrated is called a small group here. Thestructure in FIG. 6A is actually an octahedral structure, but isillustrated as a planar structure for simplicity. Note that threetetracoordinate O atoms exist in each of an upper half and a lower halfin FIG. 6A. In the small group illustrated in FIG. 6A, electric chargeis 0.

FIG. 6B illustrates a structure including one pentacoordinate Ga atom,three tricoordinate oxygen atoms (tricoordinate O atoms) close to the Gaatom, and two tetracoordinate O atoms close to the Ga atom. All thetricoordinate O atoms exist in the a-b plane. One tetracoordinate O atomexists in each of an upper half and a lower half in FIG. 6B. An In atomcan have the structure illustrated in FIG. 6B because the In atom canhave five ligands. In a small group illustrated in FIG. 6B, electriccharge is 0.

FIG. 6C illustrates a structure including one tetracoordinate Zn atomand four tetracoordinate O atoms close to the Zn atom. In FIG. 6C, onetetracoordinate O atom exists in an upper half and three tetracoordinateO atoms exists in a lower half. Alternatively, three tetracoordinate Oatoms may exist in the upper half and one tetracoordinate O atom mayexist in the lower half in FIG. 6C. In a small group illustrated in FIG.6C, electric charge is 0.

FIG. 6D illustrates a structure including one hexacoordinate Sn atom andsix tetracoordinate O atoms close to the Sn atom. In FIG. 6D, threetetracoordinate O atoms exists in each of an upper half and a lowerhalf. In a small group illustrated in FIG. 6D, electric charge is +1.

FIG. 6E illustrates a small group including two Zn atoms. In FIG. 6E,one tetracoordinate O atom exists in each of an upper half and a lowerhalf. In the small illustrated in FIG. 6E, electric charge is −1.

Here, a plurality of small groups form a medium group, and a pluralityof medium groups form a large group (also referred to as a unit cell).

Here, a rule of bonding the small groups to each other is described. Thethree O atoms in the upper half with respect to the hexacoordinate Inatom in FIG. 6A each have three proximity In atoms in the downwarddirection, and the three O atoms in the lower half each have threeproximity In atoms in the upward direction. The one O atom in the upperhalf with respect to the pentacoordinate Ga atom in FIG. 6B has oneproximity Ga atom in the downward direction, and the one O atom in thelower half has one proximity Ga atom in the upward direction. The one Oatom in the upper half with respect to the tetracoordinate Zn atom inFIG. 6C has one proximity Zn atom in the downward direction, and thethree O atoms in the lower half each have three proximity Zn atoms inthe upward direction. In this manner, the number of the tetracoordinateO atoms above the metal atom is equal to the number of the proximitymetal atoms below the tetracoordinate O atoms. Similarly, the number ofthe tetracoordinate O atoms below the metal atom is equal to the numberof the proximity metal atoms above the tetracoordinate O atoms. Sincethe coordination number of the O atom is 4, the sum of the number of theproximity metal atoms below the O atom and the number of the proximitymetal atoms above the O atom is 4. Accordingly, when the sum of thenumber of the tetracoordinate O atoms above the metal atom and thenumber of the tetracoordinate O atoms below another metal atom is 4, thetwo kinds of small groups including the metal atoms can be bonded toeach other. For example, in the case where a hexacoordinate metal (In orSn) atom is bonded through three tetracoordinate O atoms in the lowerhalf, the hexacoordinate metal atom is bonded to a pentacoordinate metal(Ga or In) atom or a tetracoordinate metal (Zn) atom.

A metal atom having the above coordination number is bonded to anothermetal atom through a tetracoordinate O atom in the c-axis direction.Further, a plurality of small groups is bonded to each other so that thetotal electric charge in a layer structure is 0. Thus, a medium group isconstituted.

FIG. 7A illustrates a model of a medium group included in a layerstructure of an In—Sn—Zn-based oxide. FIG. 7B illustrates a large groupincluding three medium groups. Note that FIG. 7C illustrates atomicorder in the case of the layer structure in FIG. 7B observed from thec-axis direction.

In FIG. 7A, for simplicity, a tricoordinate O atom is omitted and thenumber of tetracoordinate O atoms is illustrated. For example, threetetracoordinate O atoms existing in each of an upper half and a lowerhalf with respect to a Sn atom are denoted by circled 3. Similarly, onetetracoordinate O atom existing in each of an upper half and a lowerhalf with respect to an In atom is denoted by circled 1. Similarly, a Znatom close to one tetracoordinate O atom in a lower half and threetetracoordinate O atoms in an upper half, and a Zn atom close to onetetracoordinate O atom in an upper half and three tetracoordinate Oatoms in a lower half are denoted.

In the medium group included in the layer structure of theIn—Sn—Zn-based oxide in FIG. 7A, in the order starting from the top, aSn atom close to three tetracoordinate O atoms in each of an upper halfand a lower half is bonded to an In atom close to one tetracoordinate Oatom in each of an upper half and a lower half, the In atom is bonded toa Zn atom close to three tetracoordinate O atoms in an upper half, theZn atom is bonded to an In atom close to three tetracoordinate O atomsin each of an upper half and a lower half through one tetracoordinate Oatom in a lower half with respect to the Zn atom, the In atom is bondedto a small group that includes two Zn atoms and is close to onetetracoordinate O atom in an upper half, and the small group is bondedto a Sn atom close to three tetracoordinate O atoms in each of an upperhalf and a lower half through one tetracoordinate O atom in a lower halfwith respect to the small group. A plurality of such medium groups isbonded to each other so that a large group is constituted.

Here, electric charge for one bond of a tricoordinate O atom andelectric charge for one bond of a tetracoordinate O atom can be assumedto be −0.667 and −0.5, respectively. For example, electric charge of ahexacoordinate or pentacoordinate In atom, electric charge of atetracoordinate Zn atom, and electric charge of a pentacoordinate orhexacoordinate Sn atom are +3, +2, and +4, respectively. Thus, electriccharge of a small group including a Sn atom is +1. Consequently, astructure having an electric charge of −1, which cancels an electriccharge of +1, is needed to form a layer structure including a Sn atom.As a structure having an electric charge of −1, the small groupincluding two Zn atoms as illustrated in FIG. 6E can be given. Forexample, when one small group including two Zn atoms is provided for onesmall group including a Sn atom, electric charge is canceled, so thatthe total electric charge in the layer structure can be 0.

Specifically, when a large group illustrated in FIG. 7B is repeated, acrystal of an In—Sn—Zn-based oxide (In₂SnZn₃O₈) can be obtained. Notethat the layer structure of the obtained In—Sn—Zn-based oxide can beexpressed as a composition formula, In₂SnZn₂O₇(ZnO)_(m) (m is 0 or anatural number).

The above rule also applies to the following oxides: a quaternary metaloxide such as an In—Sn—Ga—Zn-based oxide; a ternary metal oxide such asan In—Ga—Zn-based oxide (also referred to as IGZO), an In—Al—Zn-basedoxide, a Sn—Ga—Zn-based oxide, an Al—Ga—Zn-based oxide, a Sn—Al—Zn-basedoxide, an In—Hf—Zn-based oxide, an In—La—Zn-based oxide, anIn—Ce—Zn-based oxide, an In—Pr—Zn-based oxide, an In—Nd—Zn-based oxide,an In—Sm—Zn-based oxide, an In—Eu—Zn-based oxide, an In—Gd—Zn-basedoxide, an In—Tb—Zn-based oxide, an In—Dy—Zn-based oxide, anIn—Ho—Zn-based oxide, an In—Er—Zn-based oxide, an In—Tm—Zn-based oxide,an In—Yb—Zn-based oxide, or an In—Lu—Zn-based oxide; a binary metaloxide such as an In—Zn-based oxide, a Sn—Zn-based oxide, an Al—Zn-basedoxide, a Zn—Mg-based oxide, a Sn—Mg-based oxide, an In—Mg-based oxide,or an In—Ga-based oxide; and the like.

As an example, FIG. 8A illustrates a model of a medium group included ina layer structure of an In—Ga—Zn-based oxide.

In the medium group included in the layer structure of theIn—Ga—Zn-based oxide in FIG. 8A, in the order starting from the top, anIn atom close to three tetracoordinate O atoms in each of an upper halfand a lower half is bonded to a Zn atom close to one tetracoordinate Oatom in an upper half, the Zn atom is bonded to a Ga atom close to onetetracoordinate O atom in each of an upper half and a lower half throughthree tetracoordinate O atoms in a lower half with respect to the Znatom, and the Ga atom is bonded to an In atom close to threetetracoordinate O atoms in each of an upper half and a lower halfthrough one tetracoordinate O atom in a lower half with respect to theGa atom. A plurality of such medium groups is bonded to each other sothat a large group is constituted.

FIG. 8B illustrates a large group including three medium groups. Notethat FIG. 8C illustrates atomic order in the case of the layer structurein FIG. 8B observed from the c-axis direction.

Here, since electric charge of a hexacoordinate or pentacoordinate Inatom, electric charge of a tetracoordinate Zn atom, and electric chargeof a pentacoordinate Ga atom are +3, +2, and +3, respectively, electriccharge of a small group including any of an In atom, a Zn atom, and a Gaatom is 0. Thus, the total electric charge of a medium group having acombination of such small groups is always 0.

Further, a medium group forming the stacked structure of theIn—Ga—Zn-based oxide is not limited to the medium group illustrated inFIG. 8A. A large group can be formed including a combination of aplurality of medium groups in which the arrangement of the In atom, theGa atom, and the Zn atom is different from that in FIG. 8A.

When an In—Sn—Zn-based oxide is formed, an oxide target having acomposition ratio: In:Sn:Zn=1:2:2, 2:1:3, 1:1:1, or 20:45:35 in anatomic ratio may be used.

The thickness of the oxide semiconductor film 110 is preferably greaterthan or equal to 3 nm and less than or equal to 50 nm.

Next, the second etching mask 112 is formed over the oxide semiconductorfilm 110 (FIG. 2A).

Here, when the base insulating film 102 is formed using an insulatingoxide which functions as an oxygen supply source, heat treatment whichsupplies oxygen to the oxide semiconductor film 110 may be performedafter the oxide semiconductor film 110 is formed and before the secondetching mask 112 is formed.

The second etching mask 112 may be formed using a resist material. Notethat there is no limitation thereto as long as it functions as a maskwhen the oxide semiconductor film 110 is processed.

Next, the oxide semiconductor film 110 is processed with the use of thesecond etching mask 112, so that the oxide semiconductor film 114 isformed (FIG. 2B).

The oxide semiconductor film 110 may be processed by dry etching. Forexample, a chlorine gas or a mixed gas of a boron trichloride gas and achlorine gas can be given as an etching gas used for the dry etching.However, there is no limitation thereto; wet etching may be used oranother method may be used.

Next, the second etching mask 112 is removed, and the first insulatingfilm 116 is formed so as to cover at least the oxide semiconductor film114 (FIG. 2C).

In the case where the second etching mask 112 is formed using a resistmaterial, the second etching mask 112 may be removed by ashing usingoxygen plasma. Alternatively, a resist stripper may be used. Further,alternatively, both of them may be used.

The first insulating film 116 is formed using an insulating oxide whichfunctions as an oxygen supply source, and may be formed using siliconoxide, silicon oxynitride, or silicon nitride oxide, for example.

The thickness of the first insulating film 116 may be greater than orequal to 1 nm and less than or equal to 30 nm, preferably greater thanor equal to 3 nm and less than or equal to 10 nm.

The first insulating film 116 may be formed by a sputtering method.

Here, the first insulating film 116 is formed using an insulating oxidewhich functions as an oxygen supply source. Therefore, heat treatmentwhich supplies oxygen to the oxide semiconductor film 114 is preferablyperformed after the first insulating film 116 is formed. By performingheat treatment which supplies oxygen to the oxide semiconductor film114, the number of oxygen defects can be reduced.

The first insulating film 116 functions as at least a gate insulatingfilm.

Next, the second conductive film 120 is formed over the first insulatingfilm 116 (FIG. 3A).

The second conductive film 120 may be formed using a material and amethod similar to those of the first conductive film 104.

Next, the third etching mask 122 is formed over the second conductivefilm 120 (FIG. 3B).

The third etching mask 122 may be formed using a resist material.However, there is no limitation thereto as long as it functions as amask when the second conductive film 120 is processed.

Next, the second conductive film 120 is processed with the use of thethird etching mask 122, so that the second conductive film 124 is formed(FIG. 3C).

The second conductive film 120 may be processed by dry etching. Forexample, a chlorine gas or a mixed gas of a boron trichloride gas and achlorine gas can be given as an etching gas used for the dry etching.However, there is no limitation thereto; wet etching may be used oranother method may be used.

The second conductive film 124 functions as at least a gate electrode.

Next, the third etching mask 122 is removed and a dopant is added to theoxide semiconductor film 114 using the second conductive film 124 as amask, so that the oxide semiconductor film 126 including the sourceregion and the drain region is formed (FIG. 4A). The oxide semiconductorfilm 126 includes a region 126A which is one of the source region andthe drain region, a region 126B which functions as a channel formationregion, and a region 126C which is the other of the source region andthe drain region.

In the case where the third etching mask 122 is formed using a resistmaterial, the third etching mask 122 may be removed by ashing usingoxygen plasma. Alternatively, a resist stripper may be used. Further,alternatively, both of them may be used.

The dopant to the oxide semiconductor film 114 may be added by an ionimplantation method or an ion doping method. Alternatively, the dopantmay be added by performing plasma treatment in an atmosphere of a gascontaining the dopant. As the added dopant, phosphorus, arsenic, or thelike may be used.

Note that dopant may be added in the state where the third etching mask122 is formed. Alternatively, dopant may be added after a secondinsulating film 118 which is described below is formed.

Next, the second insulating film 118 is formed over the first insulatingfilm 116 and the second conductive film 124, and a fourth etching mask128 is formed over a portion of the second insulating film 118 otherthan the portion of the second insulating film 118 over which theopening portion is formed (FIG. 4B).

The second insulating film 118 is formed using a material which has ahigh gas barrier property and is not easily etched. As such a material,an aluminum oxide can be used, for example.

The thickness of the second insulating film 118 may be greater than orequal to 30 nm, and may be preferably greater than or equal to 50 nm.

The second insulating film 118 may be formed by a CVD method or asputtering method.

The fourth etching mask 128 may be formed using a resist material. Notethat the material of the fourth etching mask 128 is not limited theretoas long as the material functions as a mask when the first insulatingfilm 116 and the second insulating film 118 are processed.

Next, a portion of the second insulating film 118 and a portion of thefirst insulating film 116, which are not covered with the fourth etchingmask 128, are processed to form an opening portion 130 (FIG. 4C). Sincethe second insulating film 118 is formed using a material which is noteasily etched, in the opening portion 130, an inclination angle betweena side surface of the second insulating film 118 and the substratesurface is smaller than an inclination angle between a side surface ofthe first insulating film 116 and the substrate surface.

The second insulating film 118 and the first insulating film 116 may beprocessed by dry etching. For example, a chlorine gas or a mixed gas ofa boron trichloride gas and a chlorine gas can be given as an etchinggas used for the dry etching. However, there is no limitation thereto;wet etching may be used or another method may be used.

Argon plasma treatment may be performed in the state in FIG. 4C (FirstMode). Instead of argon, another rare gas (such as helium, krypton, orxenon) may be used. By argon plasma treatment, unevenness of the surfaceof the opening portion 130 is removed, and planarity of the surface isimproved.

Then, the fourth etching mask 128 is removed (FIG. 5A).

In the case where the fourth etching mask 128 is formed using a resistmaterial, the fourth etching mask 128 may be removed by ashing usingoxygen plasma. Alternatively, a resist stripper may be used. Further,alternatively, both of them may be used.

Next, a third conductive film 132 is formed over the second insulatingfilm 118 so as to connect with the first conductive film 108 in theopening portion 130, and a fifth etching mask 134 is formed over thethird conductive film 132 (FIG. 5B).

The third conductive film 132 may be formed using a material and amethod similar to those of the first conductive film 104 and the secondconductive film 120. However, in the case where the semiconductor deviceis a display device, the third conductive film 132 may be formed using alight-transmitting conductive material such as indium tin oxide, indiumoxide containing tungsten oxide, indium zinc oxide containing tungstenoxide, indium oxide containing titanium oxide, indium tin oxidecontaining titanium oxide, or indium tin oxide to which indium zincoxide or silicon oxide is added.

In the case where unevenness of the surface of the opening portion 130is removed by argon plasma treatment, defective formation of the thirdconductive film 132 can be prevented.

The fifth etching mask 134 may be formed of a resist material. However,there is no limitation thereto as long as it functions as a mask whenthe third conductive film 132 is processed.

In the case where the third conductive film 132 is formed by asputtering method, reverse sputtering is preferably performed on theopening portion 130 (Second Mode). This is because the reversesputtering can be performed in a sputtering apparatus, and etchingresidues and the like in the opening portion 130 can be removed withoutlowering throughput. The reverse sputtering is preferably performed inthe state illustrated in FIG. 5A. This is because, when the reversesputtering is performed before the fourth etching mask 128 is removed,the substrate 100 has to be carried into the sputtering apparatus againafter the reverse sputtering is performed, and throughput is lowered.

When unevenness of the surface of the opening portion 130 is removed bythe reverse sputtering, defective formation of the third conductive film132 can be prevented.

Next, the third conductive film 132 is processed using the fifth etchingmask 134 to form a third conductive film 136. Then, the fifth etchingmask 134 is removed (FIG. 5C).

The third conductive film 132 may be processed by dry etching or wetetching.

In the case where the fifth etching mask 134 is formed using a resistmaterial, the fifth etching mask 134 may be removed by ashing usingoxygen plasma. Alternatively, a resist stripper may be used. Further,alternatively, both of them may be used.

In the manner described above, the semiconductor device of thisembodiment can be manufactured.

Note that the material of the first insulating film 116 and the secondinsulating film 118 may be reversed. At this time, since the firstinsulating film 116 is formed using a material which is not easilyetched, in the opening portion 130, the inclination angle between theside surface of the second insulating film 118 and the substrate surfaceis larger than that between the side surface of the first insulatingfilm 116 and the substrate surface.

Embodiment 2

Although the case where an opening portion is formed in a stacked-layerinsulating film having a two-layer structure is described in Embodiment1, a stacked-layer insulating film in which an opening portion is formedmay have a three-layer structure. In this embodiment, the case where astacked-layer insulating film in which an opening portion is formed hasa three-layer structure will be described. That is, a passivation filmis formed before the opening portion is formed, and another openingportion may also be formed in the passivation film.

First, in a manner similar to that in Embodiment 1, a base insulatingfilm 202 is formed over a substrate 200, a first conductive film 204 isformed over the base insulating film 202, an oxide semiconductor film206 is formed in contact with the first conductive film 204, a firstinsulating film 208 is formed to cover the oxide semiconductor film 206,a second conductive film 212 is formed over the first insulating film208, and a second insulating film 210 and a third insulating film 214are formed over the first insulating film 208 so as to cover the secondconductive film 212. Then, a first etching mask 216 is formed over aportion of the third insulating film 214 other than the portion overwhich the opening portion is formed (FIG. 9A).

The oxide semiconductor film 206 includes a region 206A which is one ofa source region and a drain region, a region 206B which is a channelformation region, and a region 206C which is the other of the sourceregion and the drain region.

The substrate 200 corresponds to the substrate 100 in Embodiment 1. Thebase insulating film 202 corresponds to the base insulating film 102 inEmbodiment 1.

The first conductive film 204 corresponds to the first conductive film108 in Embodiment 1.

The oxide semiconductor film 206 corresponds to the oxide semiconductorfilm 126 in Embodiment 1.

The first insulating film 208 corresponds to the first insulating film116 in Embodiment 1. Thus, the first insulating film 208 is formed by asputtering method using an insulating oxide which functions as an oxygensupply source (e.g., silicon oxide, silicon oxynitride, or siliconnitride oxide).

Here, the first insulating film 208 is formed using an insulating oxidewhich functions as an oxygen supply source. Therefore, after the firstinsulating film 208 is formed, heat treatment which supplies oxygen tothe oxide semiconductor film 206 is preferably performed.

The second insulating film 210 corresponds to the second insulating film118 in Embodiment 1. That is, the second insulating film 210 is formedusing a material which has a high gas barrier property and is lesseasily etched than the first insulating film 208 and the thirdinsulating film 214.

The second conductive film 212 corresponds to the second conductive film124 in Embodiment 1.

The third insulating film 214 is formed using a material and a methodsimilar to those of the first insulating film 208. The third insulatingfilm 214 functions as a passivation film. There is no particularlimitation on the thickness of the third insulating film 214, but thethickness of the third insulating film 214 is preferably greater than orequal to 100 nm.

The first etching mask 216 corresponds to the fourth etching mask 128 inEmbodiment 1.

Next, the portions of the third insulating film 214, the secondinsulating film 210, and the first insulating film 208, which are notcovered with the first etching mask 216 are processed so that an openingportion 218 is formed (FIG. 9B). Since the second insulating film 210 isformed using a material which is not easily etched, in the openingportion 218, the inclination angles between a side surface of the firstinsulating film 208 and the substrate surface and between a side surfaceof the third insulating film 214 and the substrate surface are largerthan an inclination angle between a side surface of the secondinsulating film 210 and the substrate surface.

The opening portion 218 may be formed by dry etching. For example, achlorine gas or a mixed gas of a boron trichloride gas and a chlorinegas can be given as an etching gas used for the dry etching. However,there is no limitation thereto; wet etching may be used or anothermethod may be used.

Argon plasma treatment may be performed in the state illustrated in FIG.9B (First Mode). Instead of argon, another rare gas (such as helium,krypton, or xenon) may be used. By argon plasma treatment, unevenness ofthe surface of the opening portion 218 is removed, and planarity of thesurface is improved.

Next, the first etching mask 216 is removed, a third conductive film 220is formed over the third insulating film 214 so as to connect with thefirst conductive film 204 in the opening portion 218, and a secondetching mask 222 is formed over the third conductive film 220 (FIG. 9C).

In the case where the third conductive film 220 is formed by asputtering method, reverse sputtering is preferably performed on theopening portion 218 (Second Mode). This is because the reversesputtering can be performed in a sputtering apparatus, and etchingresidues and the like in the opening portion 218 can be removed withoutlowering throughput. The reverse sputtering is preferably performedafter the first etching mask 216 is removed, before the third conductivefilm 220 is formed. This is because, when the reverse sputtering isperformed before the first etching mask 216 is removed, the substrate200 has to be carried into the sputtering apparatus again after thereverse sputtering is performed, and throughput is lowered.

After that, the third conductive film 220 is processed using the secondetching mask 222, and the second etching mask 222 is removed (notillustrated).

In the manner described above, the semiconductor device of thisembodiment can be manufactured.

In the case where an opening portion is formed in a stacked-layerinsulating film having a three-layer structure as described in thisembodiment, the shape of the opening portion varies depending on thethickness of the first to the third insulating films.

FIG. 10A shows a shape of an opening portion in the case where thethickness of the first insulating film is similar to that of the thirdinsulating film. An opening portion 308 is formed in a first insulatingfilm 302, a second insulating film 304, and a third insulating film 306provided over a formation surface 300, and in the opening portion 308,an inclination angle between a side surface of the second insulatingfilm 304 and the formation surface 300 is smaller than inclinationangles between a side surface of the first insulating film 302 and theformation surface 300 and between a side surface of the third insulatingfilm 306 and the formation surface 300.

FIG. 10B shows a shape of an opening portion in the case where the firstinsulating film is thinner than the third insulating film. An openingportion 318 is formed in a first insulating film 312, a secondinsulating film 314, and a third insulating film 316 provided over aformation surface 310, and in the opening portion 318, an inclinationangle between a side surface of the second insulating film 314 and theformation surface 310 is smaller than inclination angles between a sidesurface of the first insulating film 312 and the formation surface 310and between a side surface of the third insulating film 316 and theformation surface 310.

FIG. 10C shows a shape of an opening portion in the case where the firstinsulating film is thicker than the third insulating film. An openingportion 328 is formed in a first insulating film 322, a secondinsulating film 324, and a third insulating film 326 provided over aformation surface 320, and in the opening portion 328, an inclinationangle between a side surface of the second insulating film 324 and theformation surface 320 is smaller than inclination angles between theside surface of the first insulating film 322 and the formation surface320 and between a side surface of the third insulating film 326 and theformation surface 320.

In FIGS. 10A to 10C, the second insulating film is formed using amaterial which is not easily etched; therefore, it takes longer time toetch the second insulating film than to etch the first insulating filmor the third insulating film. Therefore, when the second insulating filmis etched, the larger the exposed portion of the opening portion is, themore remarkable a phenomenon in which a particle and the like generatedby etching are attached to the opening portion (redeposition) is. InFIG. 10B, when the second insulating film which is not easily etched isetched, the redeposition is caused in a wide range of the openingportion.

Therefore, in the case where the first insulating film is thinner thanthe third insulating film as illustrated in FIG. 10B, the effect ofargon plasma treatment or reverse sputtering is particularly remarkable.For this reason, in the semiconductor device in the present invention,it is preferable that the first insulating film is thinner than thethird insulating film.

However, the present invention is not limited thereto; the thickness ofthe first insulating film may be similar to that of the third insulatingfilm as illustrated in FIG. 10A, or the first insulating film may bethicker than the third insulating film as illustrated in FIG. 10C.

In this embodiment, a top-gate bottom-contact transistor is illustratedand described; however, the present invention is not limited thereto.The transistor may be a top-gate top-contact transistor, a bottom-gatebottom-contact transistor, or a bottom-gate top-contact transistor.

Example 1

In this example, a comparative example in which a three-layer insulatingfilm is formed and an opening portion is formed in the three-layerinsulating film and an example which is one embodiment of the presentinvention and in which the opening portion is subjected to argon plasmatreatment in the state where the opening portion is exposed are comparedand described.

First, a base insulating film 202 was formed using silicon oxynitrideover a substrate 200 by a plasma CVD method. Its thickness was 100 nm.

Next, a first conductive film 204 was formed using tungsten over thebase insulating film 202 by a sputtering method. Its thickness was 150nm.

Next, a first insulating film 208 was formed using silicon oxynitrideover the first conductive film 204 by a plasma CVD method. Its thicknesswas 30 nm.

Next, a second insulating film 210 was formed using aluminum oxide by asputtering method. Its thickness was 100 nm.

Next, a third insulating film 214 was formed using silicon oxynitride bya plasma CVD method. Its thickness was 300 nm.

Then, a first etching mask 216 was formed over the third insulating film214. In the state where the first etching mask 216 was formed, the thirdinsulating film 214, the second insulating film 210, and the firstinsulating film 208 were etched and an opening portion 218 was formed.The etching was performed in the following conditions.

-   Flow rate of trifluoromethane (CHF₃) gas: 22.5 sccm-   Flow rate of helium (He) gas: 127.5 sccm-   Flow rate of methane (CH₄) gas: 5 sccm-   Pressure in reaction chamber: 5.0 Pa-   ICP power: 475 W-   Bias power between upper electrode and lower electrode: 300 W-   Substrate temperature: 70° C.

Next, only Example Sample was subjected to argon plasma treatment in thestate where the opening portion 218 was exposed. The plasma treatmentwas performed in the following conditions.

-   Flow rate of argon (Ar) gas: 100 sccm-   Pressure in reaction chamber: 1.35 Pa-   ICP power: 500 W-   Bias power between upper electrode and lower electrode: 100 W-   Substrate temperature: −10° C.

After that, the first etching mask 216 was removed, and a stacked-layerconductive film was formed in the opening portion 218 by a sputteringmethod. The stacked-layer conductive film has a stacked-layer structureincluding a titanium layer, a tungsten layer, an aluminum layer, and atitanium layer having thicknesses of 20 nm, 50 nm, 200 nm, and 50 nm,respectively.

FIGS. 11A and 11B show cross-sectional STEM (scanning transmissionelectron microscope) images of Example Sample and Comparative Sample inwhich a stacked-layer conductive film was formed in the opening portion218. FIG. 11A shows Comparative Sample and FIG. 11B shows ExampleSample.

From the comparison between FIG. 11A and FIG. 11B, it is found that inthe sample shown in FIG. 11B which is subjected to argon plasmatreatment, unevenness of the surface of the opening portion 218 wasremoved, and planarity of the surface is high. Therefore, astacked-layer conductive film is favorably formed with high uniformity.

Further, when contact resistance in Example Sample and ComparativeSample is measured, at contact diameters of 0.4 μm and 0.5 μm, variationis extremely large in Comparative Sample, while variation is small inExample Sample. Furthermore, in Example Sample, the contact resistancetends to converge at the neighborhood of the lowest resistance inComparative Sample.

This application is based on Japanese Patent Application serial no.2011-135024 filed with Japan Patent Office on Jun. 17, 2011, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A semiconductor device comprising: an oxidesemiconductor film that comprises a source region, a drain region and achannel formation region between the source region and the drain region,wherein each of the source region and the drain region comprises animpurity; an electrode electrically connected to the oxide semiconductorfilms, wherein the electrode is one of a source electrode and a drainelectrode; a gate electrode over the oxide semiconductor film, the gateelectrode overlapping with the channel formation region; a firstinsulating film over the oxide semiconductor film and the electrode,wherein the first insulating film is capable of supplying oxygen intothe oxide semiconductor film, and is located between the oxidesemiconductor film and the gate electrode; a second insulating film overthe first insulating film; and a conductive film over the secondinsulating film, wherein the conductive film is electrically connectedto the electrode through an opening of the first insulating film and anopening of the second insulating film, wherein a first angle between aside surface of the first insulating film and a bottom surface of thefirst insulating film at the opening of the first insulating film islarger than a second angle between a side surface of the secondinsulating film and a bottom surface of the second insulating film atthe opening of the second insulating film, wherein the side surface ofthe first insulating film and the side surface of the second insulatingfilm are in contact with the conductive film at the respective openings,and wherein the oxide semiconductor film comprises non-single crystalsof an In—Ga—Zn-based oxide in which a c-axis is aligned in a directionsubstantially perpendicular to a surface of the oxide semiconductorfilm, wherein an a-axis of one of the non-single crystals is differentfrom an a-axis of another of the non-single crystals, or a b-axis of oneof the non-single crystals is different from a b-axis of another of thenon-single crystals.
 2. The semiconductor device according to claim 1,further comprising: a third insulating film over the second insulatingfilm, wherein a third angle between a side surface of the thirdinsulating film and a bottom surface of the third insulating film islarger than the second angle.
 3. The semiconductor device according toclaim 1, wherein the first insulating film comprises one of a siliconoxide, a silicon oxynitride, and a silicon nitride oxide, and whereinthe second insulating film comprises aluminum oxide.
 4. Thesemiconductor device according to claim 1, wherein a thickness of thefirst insulating film is thinner than the gate electrode.
 5. Asemiconductor device comprising: an oxide semiconductor film thatcomprises a source region, a drain region and a channel formation regionbetween the source region and the drain region, wherein each of thesource region and the drain region comprises an impurity; a sourceelectrode and a drain electrode which are electrically connected to theoxide semiconductor film; a gate electrode over the oxide semiconductorfilm, the gate electrode overlapping with the channel formation region;a first insulating film over the oxide semiconductor film, the sourceelectrode, and the drain electrode, wherein the first insulating film iscapable of supplying oxygen into the oxide semiconductor film, and islocated between the oxide semiconductor film and the gate electrode; asecond insulating film over the first insulating film and the gateelectrode; and a conductive film over the second insulating film,wherein the conductive film is electrically connected to one of thesource electrode and the drain electrode through an opening of the firstinsulating film and an opening of the second insulating film, wherein afirst angle between a side surface of the first insulating film and abottom surface of the first insulating film at the opening of the firstinsulating film is larger than a second angle between a side surface ofthe second insulating film and a bottom surface of the second insulatingfilm at the opening of the second insulating film, wherein the sidesurface of the first insulating film and the side surface of the secondinsulating film are in contact with the conductive film at therespective openings, and wherein the oxide semiconductor film comprisesnon-single crystals of an In—Ga—Zn-based oxide in which a c-axis isaligned in a direction substantially perpendicular to a surface of theoxide semiconductor film, wherein an a-axis of one of the non-singlecrystals is different from an a-axis of another of the non-singlecrystals, or a b-axis of one of the non-single crystals is differentfrom a b-axis of another of the non-single crystals.
 6. Thesemiconductor device according to claim 5, further comprising: a thirdinsulating film over the second insulating film, wherein a third anglebetween a side surface of the third insulating film and a bottom surfaceof the third insulating film is larger than the second angle.
 7. Thesemiconductor device according to claim 5, wherein the first insulatingfilm comprises one of a silicon oxide, a silicon oxynitride, and asilicon nitride oxide, and wherein the second insulating film comprisesaluminum oxide.
 8. The semiconductor device according to claim 5,wherein a thickness of the first insulating film is thinner than thegate electrode.
 9. The semiconductor device according to claim 5,wherein a concentration of the impurity in each of the source region andthe drain region is larger than a concentration of the impurity in thechannel formation region.
 10. A semiconductor device comprising: anoxide semiconductor film that comprises a source region, a drain regionand a channel formation region between the source region and the drainregion, wherein each of the source region and the drain region comprisesan impurity; an electrode electrically connected to the oxidesemiconductor film, wherein the electrode is one of a source electrodeand a drain electrode; a gate electrode over the oxide semiconductorfilm, the gate electrode overlapping with the channel formation region;a first insulating film over the oxide semiconductor film and theelectrode, wherein the first insulating film is capable of supplyingoxygen into the oxide semiconductor film, and is located between theoxide semiconductor film and the gate electrode; a second insulatingfilm over the first insulating film; and a conductive film over thesecond insulating film, wherein the conductive film is electricallyconnected to the electrode through an opening of the first insulatingfilm and an opening of the second insulating film, wherein a first anglebetween a side surface of the first insulating film and a bottom surfaceof the first insulating film at the opening of the first insulating filmis larger than a second angle between a side surface of the secondinsulating film and a bottom surface of the second insulating film atthe opening of the second insulating film, wherein the side surface ofthe first insulating film and the side surface of the second insulatingfilm are in contact with the conductive film at the respective openings,wherein the oxide semiconductor film comprises non-single crystals of anIn—Ga—Zn-based oxide in which a c-axis is aligned in a directionsubstantially perpendicular to a surface of the oxide semiconductorfilm, wherein an a-axis of one of the non-single crystals is differentfrom an a-axis of another of the non-single crystals, or a b-axis of oneof the non-single crystals is different from a b-axis of another of thenon-single crystals, and wherein the impurity is one of phosphorus andarsenic.
 11. The semiconductor device according to claim 10, furthercomprising: a third insulating film over the second insulating film,wherein a third angle between a side surface of the third insulatingfilm and a bottom surface of the third insulating film is larger thanthe second angle.
 12. The semiconductor device according to claim 10,wherein the first insulating film comprises one of a silicon oxide, asilicon oxynitride, and a silicon nitride oxide, and wherein the secondinsulating film comprises aluminum oxide.
 13. The semiconductor deviceaccording to claim 10, wherein a thickness of the first insulating filmis thinner than the gate electrode.